Steel for induction hardening with excellent cold workability, rolling member made of the same, and motion guide device using rolling member

ABSTRACT

This steel for induction hardening contains, by percent by mass, C: 0.7 to 0.9%, Si: 0.4 to 1.0%, Mn: 0.5 to 1.25%, P: 0.030% or less, S: 0.030% or less, Cr: 0.4% or less, Al: 0.05% or less, Ti: 0.003% or less, and O: 0.0020% or less, wherein [Mn]≦0.5+0.35/[Si] is met, the steel further contains Fe and unavoidable impurity and the steel has annealing hardness of 93 HRB or less, and wherein an induction hardening layer after induction hardening has residual austenite of 20 to 40% and hardness of 61 HRC or more. This makes it possible to provide steel for induction hardening in which lowering of a fatigue life hardly occurs even though foreign objects incorporates into lubricating oil of a motion guide device and concentration of stress on its dent causes flaking due to lubrication failure to be generated on a steel surface of a device, or even though rolling/sliding contact occurs.

TECHNICAL FIELD

The present invention relates to steel for induction hardening by which lowering of a fatigue life hardly occurs under lubrication failure, and further to a motion guide device including, as apart thereof, a rolling member made of the steel for induction hardening.

BACKGROUND ART

FIG. 1 is a partially outer perspective sectional view for explaining a schematic configuration of a motion guide device, and shows a linear motion guide device, incorporated into a machine tool, for guiding a table for supporting a workpiece, for example. This motion guide device is constructed from a track rail (bearing ring) 1 and a slide board (slide member) 3. Ball rolling surfaces 1 a are formed along a longitudinal direction on the track rail 1. The slide board 3 fits this track rail 1 via a number of balls 2 as rolling members (or rolling material), and includes an endless circulation passage for the balls 2 therein. The slide board 3 reciprocally moves on the track rail 1 with circulation of the balls 2. However, the slide board 3 may be caused to reciprocally move on the track rail 1 as a stationary side.

The slide board 3 described above is constructed from a substantially saddle-shaped block 5 and a pair of end plates 6. The substantially saddle-shaped block 5 has amount surface 5 a for mounting a table (not shown in the drawing) or the like. The pair of end plates 6 is fixed on both front and back end surfaces of this block 5. The endless circulation passage described above is constructed from a loaded ball rolling surface 5 b, a ball return groove 5 c and a direction changing passage (not shown in the drawing). The loaded ball rolling surface 5 b and ball return groove 5 c are formed in this block 5 so as to correspond to the ball rolling surface 1 a of the track rail 1. The direction changing passage is formed in both of the end plates 6 to communicate the loaded rolling surface 5 b with the ball return groove 5 c.

In such a motion guide device, each ball 2 is in contact with the track rail 1 substantially at a point, and each ball 2 is in contact with the slide board 3 substantially at a point. Since this contact portion repeatedly moves on the same track, a rolling load is applied to the track rail 1 and the slide board 3.

In the case where the motion guide device is used under a favorable environment, no problem occurs at the contact portion between each ball 2 and the track rail 1 and the contact portion between each ball 2 and the slide board 3. However, in the case of using it under an opened environment such as a general factory, foreign objects, such as grit and dust in a usage environment or chip generated from a machine tool during processing and the like, may adhere to the contact portions. The foreign objects adhering to the contact portions jam up or bite the ball rolling surface 1 a by operating the slide board 3. A case where a fatigue life of the track rail 1 jammed by the foreign objects is markedly lowered occurs. Further, a film thickness of lubricating oil is structurally thin in the contact portion between each ball 2 ad the track rail 1 and the contact portion between each ball 2 and the slide board 3. For this reason, rolling/sliding contact of metal occurs, and a surface area of the track rail 1 and the slide board 3 is damaged, whereby a case where a fatigue life is markedly lowered also occurs.

Conventionally, in response to this case, a method of increasing the degree of hardness of constituent material of a track rail by subjecting bearing steel such as JIS-SUJ2 to brine hardening, a method of preventing foreign objects themselves from forming dent on a surface of steel by subjecting carbon steel to carburized hardening to make a carburized hardening layer deep and increasing its hardness, a method of heightening toughness against a crack of the constituent material of a track rail by using bearing steel such as JIS-SUJ3 and JIS-SUJ5 and subjecting it to high-temperature heat treatment such as martempering, or the like has been carried out.

However, in the method of subjecting bearing steel to brine hardening, the degree of hardness becomes too high, and its toughness is thus poor. A crack is early generated from damaged portions caused by incorporation of foreign objects and rolling/sliding contact, and propagation of this crack causes flaking to be generated. Thus, its usable life has not been able to be improved. Further, there has been a significant case in practice because the method of carburized hardening and the method of high-temperature heat treatment cause productivity to be lowered to lead to cost up.

Now, an invention to improve a fatigue life by increasing the amount of residual austenite has been proposed (for example, see Patent Literature 1 described below). However, there has been a case where this invention allows the fatigue life at incorporation of foreign objects to be improved, but lowering of the fatigue life due to rolling/sliding contact cannot be improved stably.

By citing Patent Literature 1 described below as cited literature, an invention is disclosed in which a fatigue life is improved when incorporation of foreign objects is reduced by reducing the amount of residual austenite (for example, see Patent Literature 2 described below). However, there has been a case where this invention cannot stably improve the fatigue life at incorporation of foreign objects as Patent Literature 1 described below.

Moreover, technique has been proposed in which a steel bar for induction hardening with an excellent rolling fatigue life is obtained by regulating the magnitude of inclusion in addition to chemical composition and a predetermined formula for reducing a production amount of proeutectoid ferrite and a condition defining the degree of segregation and an Ms point of temperature at which a martensitic structure starts to be generated (for example, see Patent Literature 3 described below). However, in the invention disclosed in Patent Literature 3, the preferable amount of residual austenite and surface hardness after induction hardening have not been considered. In particular, in the range of C: 0.5 to 0.7% in this invention, surface hardness is low, and this makes it impossible to obtain an excellent rolling fatigue life stably. In addition, there has been a case where damage caused by incorporation of foreign objects and rolling/sliding contact cannot be inhibited stably.

Patent Literature 1: Japanese Patent Application Publication No. 11-209844

Patent Literature 2: Japanese Patent Application Publication No. 2001-165161

Patent Literature 3: Japanese Patent Application Publication No. 2004-183016

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

It is a problem to be solved by the present invention to provide steel for induction hardening in which lowering of a fatigue life hardly occurs even though foreign objects incorporates into lubricating oil to be introduced into a motion guide device or the like and concentration of stress on dent by the incorporation causes, as a major factor, flaking due to lubrication failure to be generated on a steel surface of the motion guide device, or even though rolling/sliding contact occurs because surface roughness is large and a degree of viscosity of lubricating oil is low or the amount of lubricating oil is lacking, and a film thickness of lubricating oil is thereby thin. It is another problem to be solved by the present invention to provide a rolling member made of the steel for induction hardening and a motion guide device having the rolling member.

Means for Solving the Problems

Inventors have reviewed, with all their hearts, Patent Literature 1 described above, which is a prior application of the invention of applicants of the present application. As a result, the inventors found that Si is to be contained with 0.4% or more and hardness after induction hardening is required to be 61 HRC or more in order to suppress lowering of fatigue strength (or fatigue life) due to rolling/sliding contact of metal. On the other hand, as an additive amount of Si is increased, cold workability is marred. For that reason, in order to ensure preferable cold workability, it is necessary to reduce the amount of C, the amount of Mn and the amount of Cr. However, in the case where the amount of C is reduced, hardness after induction hardening cannot be obtained sufficiently. Further, in the case where the amount of Mn is reduced, hardenability is marred. However, the inventors found that annealing hardness is lowered by reducing the amount of Cr and this makes it possible to improve cold workability significantly, and thus obtained the measure of the present invention. Namely, in order to solve the cases described above, the steel for induction hardening according to the present invention is directed to steel for induction hardening containing: with percent by mass, C: 0.7 to 0.9%; Si: 0.4 to 1.0%; Mn: 0.5 to 1.25%; P: 0.030% or less; S: 0.030% or less; Cr: 0.4% or less; Al: 0.05% or less; Ti: 0.003% or less; and O: 0.0020% or less, wherein [Mn]≦0.5+0.35/[Si] is met, and the steel further contains Fe (iron) and unavoidable impurity and the steel has annealing hardness of 93 HRB or less, and wherein an induction hardening layer after induction hardening has residual austenite of 20 to 40% and hardness of 61 HRC or more.

In the steel for induction hardening according to the present invention, chemical composition of steel has C in the range of 0.7 to 0.9% and Si in the range of 0.4 to 1.0%, and annealing hardness is 93 HRB or less. Thus, it has excellent cold workability. Further, an induction hardening layer by subjecting a product processed by cold working to induction hardening has residual austenite of 20 to 40% and high hardness of 61 HRC or more. It has excellent toughness. Moreover, a crack is hardly generated from a damaged portion caused by incorporation of foreign objects in the product and rolling/sliding contact. If a crack is generated, the generated crack hardly propagates, and flaking is hardly generated. Therefore, it is steel for induction hardening used for resistance to a foreign object environment and resistance to rolling/sliding contact, by which it is possible to improve its usable life. Moreover, since the degree of hardness of the induction hardening layer is high hardness of 61 HRC or more, it is preferable to be used as a track rail of a motion guide device. Since steel having both high toughness and high hardness can be obtained, it becomes excellent steel for induction hardening. Further, since this steel for induction hardening used for resistance to a foreign object environment and resistance to rolling/sliding contact does not need a carburizing process, it is possible to obtain steel with higher (that is, desirable) hardness and higher toughness at low cost. This is preferable for practice, in particular.

Moreover, since carbon, silicon, manganese and chrome constitute main elements in the steel for induction hardening according to the present invention, a member made of the steel for induction hardening has high hardness and high toughness, and can be utilized for a long usable life even under a foreign object environment and rolling/sliding contact. In addition, the steel for induction hardening is inexpensive and has excellent machinability and cold workability.

Further, the rolling member according to the present invention is a rolling member wherein an induction hardening layer having residual austenite of 20 to 40% and hardness of 61 HRC or more is formed by subjecting the steel for induction hardening as described above to cold working to form a shape of the rolling member and then subjecting the shape of the rolling member to induction hardening and tempering.

Moreover, it is preferable to form a motion guide device using the rolling member described above for a part of members of the motion guide device, for example, a track rail or a slide board.

In the motion guide device according to the present invention, as described above, the steel whose chemical composition has the amount of C in the range of 0.7 to 0.9% and the amount of Si in the range of 0.4 to 1.0%, and which has an induction hardening layer with high hardness of 61 HRC or more after cold working and has the amount of residual austenite of 20 to 40% is utilized for any one or both of the track rail and the slide board of the motion guide device, whereby a crack originating from a damaged portion caused by incorporation of foreign objects into lubricating oil to be used and rolling/sliding contact is hardly generated in the track rail of the motion guide device. As a result, a usable life of the motion guide device can be improved.

EFFECTS OF THE INVENTION

In the steel for induction hardening according to the present invention, since the amount of residual austenite in an induction hardening layer becomes 20 to 40%, it has excellent toughness. Even though foreign objects such as grit, dust and chip are incorporated into members of a product when the product is made, damage is hardly caused. Thus, a crack originated by the damage is hardly generated. Even though a crack is generated, the generated crack hardly propagates, and flaking is hardly generated. It is possible to improve a usable life of the product member made of this steel. Moreover, in the steel for induction hardening according to the present invention, since the degree of hardness of the induction hardening layer is high, 61 HRC or more, it is preferable for a track shaft of a motion guide device. It has high toughness and high hardness. Since, under lubrication failure, rolling/sliding contact hardly occurs and damage is hardly caused, a crack originated by the damage is hardly generated. Even though a crack is generated, the generated crack hardly propagates, and flaking is hardly generated. Since the usable life of the product member made of this steel can be improved, it becomes excellent induction hardened steel. Furthermore, this steel for induction hardening merely contains, as chemical composition, expensive Ni or Mo at the unavoidable impurity level. Since it does not need a carburized hardening process, it is inexpensive. It has an effect that induction hardened steel for resistance to a foreign object environment, which is excellent in cold workability and machinability, can be obtained.

Further, the rolling member according to the present invention has residual austenite of 20 to 40% by subjecting the steel for induction hardening according to the present invention described above to cold working to a shape of a rolling member and then subjecting it to induction hardening and tempering. Thus, it has an effect that it is preferable in view of a fatigue life and it is formed in an induction hardening layer with hardness of 61 HRC or more.

Moreover, the motion guide device according to the present invention is a motion guide device in which the rolling member according to the present invention described above is utilized as a part. Thus, this motion guide device has high hardness of 61 HRC or more, a residual austenite layer of 20 to 40% and high toughness. Even though it is utilized under a foreign object environment, a crack is hardly generated from a damaged portion caused by the foreign objects. Even though a crack is generated, the crack hardly propagates, and flaking is hardly generated. Moreover, in the motion guide device according to the present invention, rolling/sliding contact hardly occurs even under lubrication failure, and damage is hardly caused. Thus, a crack originated by the damage is hardly generated. Even though a crack is generated, the generated crack hardly propagates and flaking is hardly generated. Further, since the motion guide device according to the present invention does not contain expensive material or material that inhibits workability, it has an effect to be excellent in a long usable life and cost performance.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of a motion guide device.

BEST MODE FOR CARRYING OUT THE INVENTION

As the best mode for carrying out the present invention, each of steel for induction hardening, a rolling member (or rolling material) using the steel for induction hardening and a motion guide device including the obtained rolling member as a member will be described specifically with reference to Tables and Drawing. In this regard, the “lubrication failure” in this specification means a state in which foreign objects incorporate into lubricating oil and concentration of stress on its dent as a major factor leads to flaking, and a state in which a film thickness of the lubricating oil is thin because surface roughness is large, the degree of viscosity of the lubricating oil is low, and/or the amount of lubricating oil is lacking, or because of combination thereof, whereby rolling/sliding contact of metal occurs mainly. Further, a motion guide device according to the present invention is defined as a collective term of a linear motion guide device shown in FIG. 1 and ones in which a track shaft and a slide member are assembled mutually movably via a ball spline, a ball screw, or a rolling member such as a ball or a roller (not shown in the drawing).

First, steel for induction hardening according to the present embodiment has chemical composition containing, by percent by mass, C: 0.7 to 0.9%, Si: 0.4 to 1.0%, Mn: 0.5 to 1.25%, P: 0.030% or less, S: 0.030% or less, Cr: 0.4% or less, Al: 0.05% or less, Ti: 0.003% or less and O: 0.0020% or less; and meeting [Mn]≦0.5+0.35/[Si], wherein the remnant is made of Fe and unavoidable impurity. The steel for induction hardening composed of this composition is steel in which an induction hardening layer after induction hardening has residual austenite of 20 to 40% and hardness of 61 HRC or more.

Reasons for limitation of chemical composition of the steel for induction hardening according to the present embodiment as described above, the amount of residual austenite in the induction hardening layer and the degree of hardness will be described below. In this regard, hereinafter, percentages (%) in a range for chemical composition indicate percent by mass.

(C: 0.7 to 0.9%)

C is an Element Required to Form Carbide by Means of induction hardening and to make hardness after tempering of the induction hardening layer be 61 HRC or more. In addition, C is an element particularly effective for generating residual austenite of 20 to 40% in the induction hardening layer. However, in the case where C is regulated to less than 0.6%, desired hardness cannot be obtained in the induction hardening layer. Moreover, in the case where C is regulated to less than 0.7%, a usable life of the motion guide device made of this steel becomes shorter under an environment in which rolling/sliding contact occurs. On the other hand, in the case where C exceeds 0.9%, machinability and cold workability are deteriorated, and a hardening crack is further generated at induction hardening. Accordingly, C is regulated to 0.7 to 0.9%.

(Si: 0.4 to 1.0%)

Si is an element to be added as a deoxidizing agent. However, in the case where Si is less than 0.4%, a usable life of the motion guide device made of this steel becomes shorter under an environment in which rolling/sliding contact occurs. On the other hand, in the case where it exceeds 1.0%, machinability and cold workability are deteriorated, and the usable life becomes shorter under the environment in which rolling/sliding contact occurs. Accordingly, Si is regulated to 0.4 to 1.0%.

(Mn: 0.5 to 1.25%, and preferably 0.75 to 1.25%)

Mn is an element having a great effect to improve induction hardenability, and is an element effective for generating more residual austenite in an induction hardening layer. In the Case where Mn is regulated to less than 0.5%, it is impossible to ensure sufficient hardenability after induction hardening. On the other hand, in the case where it exceeds 1.25%, its machinability or cold workability is deteriorated. Accordingly, Mn is regulated to 0.5 to 1.25%, and preferably to 0.75 to 1.25%.

([Mn]≦0.5+0.35/[Si])

In addition to regulation of the amount of Mn described above, [Mn]≦0.5+0.35/[Si] is regulated by taking into consideration the fact that workability is deteriorated in the case where the amount of Si required to obtain a rolling fatigue life is ensured. Therefore, by limiting it by means of a relationship between the amount of Mn and the amount of Si, good workability can be obtained. Accordingly, [Mn]≦0.5+0.35/[Si] is met (or satisfied).

(Cr: 0.4% or Less, and Preferably 0.35% or Less)

Cr is an element having an effect to improve induction hardenability and toughness of steel. On the other hand, since Cr is a carbide forming element and stabilizes carbide, hardness after softening annealing is not lowered. Accordingly, Cr is regulated to 0.4% or less, and preferably to 0.35% or less.

(Al: 0.05% or less)

Al is an element to be added as a deoxidizing agent. However, in the case where Al is more than 0.05%, Al oxide is increased, whereby fatigue strength of the steel is lowered and workability is lowered. Accordingly, Al is regulated to 0.05% or less.

(Ti: 0.003% or Less)

Ti is an element effective for preventing grain coarsening. However, in the case where Ti is rich, workability is lowered. Accordingly, Ti is regulated to 0.003% or less.

(P: 0.030% or less) P is an Element Having an Effect to Improve Corrosion resistance. However, in the case where P is added so as to exceed 0.030%, it is segregated to grain boundary to promote intergranular embrittlement, and impact strength and bending strength are thus lowered. Accordingly, P is regulated to 0.030% or less.

(S: 0.030% or less)

S Bonds to MN to Become Mns, Thereby Preventing Hot shortness. However, MnS forms a nonmetallic inclusion to cause a fatigue life to be lowered. In the case where S exceeds 0.03%, lowering of the fatigue life tends to occur. Accordingly, S is regulated to 0.03% or less. In this regard, since MnS has an effect to improve machinability, it is desired that it is regulated to 0.010% or more.

(O: 0.0020% or less)

O forms oxide together with other metallic element, and segregates in grain boundary to become a factor of hot shortness. In the case where O exceeds 0.0020%, a fatigue life may be lowered. Accordingly, O is regulated to 0.0020% or less, and preferably to 0.0015% or less.

In this regard, N may further be contained in addition to the elements described above. In that case, N heightens strength by partial solution to the steel. However, in the case where N is contained too much, it encourages low-temperature brittleness markedly. Accordingly, it is desired that N is regulated to proper quantity when to be added appropriately, and its range is set to 0.01% or less.

(Residual austenite of induction hardening layer: 20 to 40%)

In the case where residual austenite of an induction hardening layer is less than 20%, a fatigue life may be lowered when incorporation of foreign objects into a steel part occurs under a foreign object environment, and a life improvement effect cannot be obtained sufficiently. The residual austenite has an effect to improve toughness as has already been known. Thus, it has an effect that rolling/sliding contact hardly occurs and lowering of the fatigue life hardly occurs even though incorporation of foreign objects such as grit, dust and chip into a track rail of the motion guide device occurs under an environment in which the foreign objects exist, for example. For this reason, it is desired that a lower limit of the amount of residual austenite in the induction hardening layer is regulated to 20% or more, and particularly to 25% or more. On the other hand, residual austenite itself is soft. Thus, in the case where the residual austenite of the induction hardening layer exceeds 40%, lowering of the fatigue life does not occur in the foreign object environment. However, since surface hardness of induction hardened steel is hardly kept at 61 HRC, this leads to lowering of the fatigue life due to rolling/sliding contact. Accordingly, it is preferable that an upper limit of the residual austenite in the induction hardening layer is regulated to 40% or less, and particularly to 35% or less.

Surface hardness of induction hardened steel: 61 HRC or more)

By regulating surface hardness of induction hardened steel to 61 HRC or more, the steel has high hardness while having high toughness. A member made of this steel can be utilized for a long usable life even under a foreign object environment and even though rolling/sliding contact occurs.

Next, a rolling member according to the present embodiment is one formed by using the steel for induction hardening according to the present embodiment described above to be subjected to cold working. In this case, since annealing hardness of the steel for induction hardening before cold working is 93 HRB or less, the cold working allows it to be formed to a desirable shape of a rolling member precisely. Moreover, this formed rolling member is subjected to induction hardening to heat and harden a surface by means of high-frequency heating at 950 to 1,050° C. for 6 to 8 seconds, and is then subjected to low-temperature tempering at tempering temperature of 130 to 180° C., preferably at 150° C. and in tempering time for 60 to 180 minutes, preferably for about 80 minutes. This becomes a rolling member on the surface of which an induction hardening layer having residual austenite of 20 to 40% and hardness of 61 HRC or more has been formed. In the above, in the case where surface heating temperature by the high-frequency heating exceeds 1,050° C., a hardening crack is readily generated. In the case where the surface heating temperature is less than 950° C., it may be impossible to ensure the residual austenite of 20% or more.

The motion guide device according to the present embodiment is one in which the rolling member according to the present embodiment described above is utilized for the track rail 1 and the slide board 3 provided on the track rail 1 of the linear motion guide device 7 shown in FIG. 1.

The motion guide device according to the present embodiment, that is, the linear motion guide device 7 has high hardness and high toughness, and is made of induction hardened steel for resistance to a foreign object environment and resistance to rolling/sliding contact in which it can be utilized with a long usable life even under a foreign object environment. Apart of members thereof, particularly the track rail 1 or the slide board 3 provided on the track rail 1 are configured by the steel for induction hardening according to the present embodiment. Therefore, even though this linear motion guide device 7 is utilized under the foreign object environment, a crack is hardly generated from a damaged portion caused by the foreign objects or a damaged portion caused by rolling/sliding contact. Further, even though a crack is generated, the generated crack hardly propagates, and flaking is hardly generated. Moreover, since this linear motion guide device 7 does not contain expensive Ni or Mo as materials with a possible contained amount, it becomes the linear motion guide device 7 excellent in cost performance with a long usable life.

Example 1

Steel containing chemical composition in each of Examples 1 to 8 and Comparative Examples 1 to 6 shown in Table 1 was melted at 100 kg VIM (vacuum induction melting furnace). This steel was heated at 1,150 to 1,200° C. to be forged (or cogging) to a round bar with φ65 mm and a round bar with φ20 mm. Each of Examples Nos. 1 to 8 in Table 1 is steel for induction hardening according to the present invention, while each of Comparative Examples Nos. 1 to 6 is comparative steel in which any component of the chemical composition drops out of the corresponding range of the present invention. By applying a shaded form thereto, the components of the chemical composition, each of which drops out of the corresponding range of the present invention, are pointed out. In this regard, a shaded portion of “0.5+0.35/Si” value in Comparative Example No. 4 indicates that it drops out of the condition of the present invention.

TABLE 1 (The unit is percentage by mass %, but the unit of O is ppm) No. C Si Mn P S Cr Al Ti O 0.5 + 0.35/Si Example 1 0.90 0.40 1.20 0.016 0.014 0.38 0.029 0.003 9 1.38 2 0.90 0.40 0.80 0.015 0.025 0.05 0.015 0.002 8 1.38 3 0.85 0.80 0.50 0.016 0.020 0.25 0.020 0.002 10 0.94 4 0.80 0.50 1.00 0.014 0.012 0.15 0.035 0.003 9 1.20 5 0.78 0.45 1.25 0.015 0.008 0.20 0.012 0.002 8 1.28 6 0.75 0.60 1.05 0.014 0.028 0.35 0.032 0.003 7 1.08 7 0.72 0.55 0.70 0.017 0.005 0.38 0.017 0.002 9 1.14 8 0.70 1.00 0.83 0.016 0.018 0.20 0.025 0.002 9 0.85 Comparative 1 1.00 0.40 1.25 0.015 0.018 0.35 0.020 0.002 8 1.38 Example 2 0.95 0.60 0.90 0.014 0.027 0.15 0.022 0.002 11 1.08 3 0.75 1.10 0.70 0.013 0.020 0.18 0.025 0.003 7 0.82 4 0.85 0.50 1.50 0.016 0.022 0.30 0.031 0.002 9 1.20 5 0.80 0.80 0.85 0.014 0.015 0.50 0.017 0.002 8 0.94 6 0.65 0.35 1.20 0.014 0.009 0.20 0.013 0.002 8 1.50 (Shaded portions drop out of the claims)

-   -   (Thrust Type Rolling Contact Fatigue Test Under Foreign-Object         Environment)

A round bar obtained by forging the steel in each of Examples and Comparative Examples shown in Table 1 to φ65 mm was subjected to normalizing at 870° C., and disk-shaped flat plates each having a thickness of 10 mm were cut out of them after subjecting them to spheroidizing annealing at 740° C. This disk-shaped flat plate with a thickness of 10 mm was subjected to high-frequency heating to keep surface temperature at 1,000° C. for 7 seconds; then subjected to polymer hardening; and tempering at 150° C. was further carried out for 80 minutes. Moreover, this hardened and tempered disk-shaped flat plate was subjected to grinding and polishing processing, and its surface is let into a mirrored state to be created in each test piece. Further, the degree of surface hardness of each of these test pieces was measured by means of a Rockwell hardness tester.

The thrust type Rolling Contact Fatigue Test under foreign object environment is a method of carrying out Rolling Contact Fatigue Test by means of a Mori thrust type rolling fatigue tester by putting foreign objects into lubricating oil. This test was carried out on conditions described below, and a rolling contact fatigue life of resistance to a foreign object environment was evaluated by L₅₀ (50% cumulative probability of fracture). In this evaluation, evaluation of each of Inventive Examples of the present application and Comparative Examples was indicated using a ratio when L₅₀ of Comparative Example No. 6 was set to one, and shown in Table 2.

Conditions of thrust type Rolling Contact Fatigue Test under foreign object environment described above:

Testing temperature: room temperature

Herzian maximum contact stress (Pmax): 540 kgf/mm2

Maximal repetition rate: 1800 cpm

Lubricant: spindle oil (#60)

Foreign object: input of powder of high-speed tool steel (SKH51) with hardness of 760 to 800 HV and particle size of 105 to 150 μm into the lubricant at the rate of one g/liter.

(Roller Pitting Test)

Each round bar forged to φ32 mm was subjected to normalizing at 870° C., and a cylinder whose rolling portion was φ26 mm were cut out of them after subjecting them to spheroidizing annealing at 740° C. The cylinder was subjected to induction hardening at surface heating temperature of 1,000° C. for 7 seconds; then subjected to polymer hardening; and tempering at 150° C. was further carried out for 90 minutes. It was subjected to grinding and polishing processing, and its surface is let into a mirrored state to be created in a test piece.

The roller pitting test is a method of carrying out a rolling/sliding contact test under high surface pressure. The test was carried out for the test piece described above on conditions described below, and a fatigue life under a rolling/sliding contact condition was evaluated by L₅₀ (50% cumulative probability of fracture).

Testing temperature: 80° C.

Herzian maximum contact stress: 3320 MPa

Loading speed: 2000 rpm

Specific sliding: −40%

Driver roller: Polish finishing one after carburization of a roller made of JIS-SCM420H steel

The fatigue life under the rolling/sliding contact condition was indicated using a ratio when L₅₀ of Comparative Example No. 6 (50% cumulative probability of fracture) was set to one, and shown in Table 2.

(Measurement of the Amount of Residual Austenite)

The amount of residual austenite of an induction hardened surface was measured with respect to a test piece manufactured in the similar manner to that of the test piece using Thrust Type Rolling Contact Fatigue Test under foreign object environment. A measuring method was an X-ray diffraction method, and test results represented by a volume ratio (%) were shown in Table 2.

(Annealing Hardness)

Annealing hardness was measured as the degree of hardness of a test surface of a test piece by means of a Rockwell hardness tester after subjecting it to spheroidizing annealing at 740° C. when the test piece for the thrust type rolling contact fatigue test under foreign object environment was manufactured. The measured results were shown in Table 2.

TABLE 2 Residual Surface L50 fatigue L50 fatigue life Annealing austenite hardness life (foreign (rolling/sliding hardness No. amount (%) (HRC) object) contact of metal) (HRB) Example 1 35 62.4 2.5 3.0 93 2 30 62.8 2.7 3.1 92 3 26 62.5 2.5 2.8 91 4 26 62.3 2.3 2.6 91 5 27 62.1 2.5 2.6 92 6 24 61.8 2.4 2.4 93 7 21 61.5 2.2 2.3 93 8 20 61.2 2.1 2.1 91 Comparative 1 41 60.5 2.3 1.2 98 Example 2 33 62.2 2.6 2.4 96 3 22 61.6 2.1 2.3 95 4 32 62.2 2.4 2.3 95 5 26 61.9 2.2 2.5 94 6 18 60.7 1.0 1.0 94 (Shaded portions drop out of the claims)

As shown in Table 2, the amount of residual austenite after the induction hardening and tempering described above fell in the range of 20 to 40% of the invention in Examples 1 to 8 according to the present invention. On the other hand, in Comparative Examples 1 to 6, Comparative Example 1 was 41% and thus exceeds the range of the present invention, and Comparative Example 6 was 18% and thus less than the range of the present invention. Moreover, surface hardness of the induction hardening layer in this case was in the range of 61.2 to 62.8 HRC in Examples 1 to 8 according to the present invention, and they met the range of 61 HRC or more according to the present invention. However, Comparative Example 1 was 60.5 HRC, and Comparative Example 6 was 60.7 HRC. Both of them did not meet the range according to the present invention. Moreover, the L₅₀ usable life under the foreign object environment fell in the range of 2.1 to 2.7 in Examples 1 to 8 according to the present invention when Comparative Example 6 is set to one as reference. Further, the L₅₀ usable life that is a fatigue life under a rolling/sliding contact environment by the roller pitting test fell in the range of 2.1 to 3.0 in Examples 1 to 8 according to the present invention when Comparative Example 6 is similarly set to one as reference, and they were higher than Comparative Examples mutually. Moreover, the annealing hardness after being subjected to spheroidizing annealing described above fell in the range of 91 to 93 HRB in Examples 1 to 8 according to the present invention, and all of them were 93 HRB or less. However, Comparative Examples 1 to 6 were 94 to 98 HRB and higher than Examples 1 to 8, and therefore, it can be seen that the steel according to the present invention has excellent cold workability. 

1. Steel for induction hardening containing: by percent by mass, C: 0.7 to 0.9%; Si: 0.4 to 1.0%; Mn: 0.5 to 1.25%; P: 0.030% or less; S: 0.030% or less; Cr: 0.4% or less; Al: 0.05% or less; Ti: 0.003% or less; and O: 0.0020% or less, wherein [Mn]≦0.5+0.35/[Si] is met, the steel further contains Fe and unavoidable impurity and the steel has annealing hardness of 93 HRB or less, and wherein an induction hardening layer after induction hardening has residual austenite of 20 to 40% and hardness of 61 HRC or more.
 2. A rolling member wherein an induction hardening layer having residual austenite of 20 to 40% and hardness of 61 HRC or more is formed by subjecting the steel for induction hardening as claimed in claim 1 to cold working to form a shape of the rolling member and then subjecting the shape of the rolling member to induction hardening and tempering.
 3. A motion guide device formed by using the rolling member as claimed in claim 2 as a part of members of the motion guide device. 